Professor Kevin Anderson, director of the Tyndall Centre for Climate Change, believes only around 10 per cent of the planet’s population – around half a billion people – will survive if global temperatures rise by 4C.

Anderson’s warning comes just eight days before global leaders meet in Copenhagen for the most crucial talks on climate change reversal since the Rio summit in 1992. Current Met Office projections reveal that the lack of action in the intervening 17 years – in which emissions of climate changing gases such as carbon dioxide have soared – has set the world on a path towards potential 4C rises as early as 2060, and 6C rises by the end of the century.

Anderson, who advises the government on climate change, said the consequences were “terrifying”.

“For humanity it’s a matter of life or death,” he said. “We will not make all human beings extinct as a few people with the right sort of resources may put themselves in the right parts of the world and survive.

“But I think it’s extremely unlikely that we wouldn’t have mass death at 4C. If you have got a population of nine billion by 2050 and you hit 4C, 5C or 6C, you might have half a billion people surviving.”

Efforts at the Copenhagen summit, which starts on 7 December, will focus on action to instead keep temperature rises to no more than 2C – generally accepted as the threshold for dangerous climate change. However, with growing pessimism that a binding agreement on emissions reduction targets will be reached, Anderson warned time was running out.

If ambitious global targets for reductions have not been set by the end of next year, he believes it will be too late to stop emissions rising beyond 2C.

Last week, Britain and France urged the wealthiest nations to set aside $10 billion annually over the next three years to help poorer countries reduce the output of greenhouse gases.

Scotland has set a 42 per cent emissions reduction target for 2020 but Anderson pointed out that even if this was achieved by rich nations throughout the world, it would only give a 60 per cent chance of avoiding a 2C global temperature increase.

Despite pessimism over the past few weeks he was optimistic a legal agreement can still be reached at Copenhagen. He believes leaders are deliberately trying to lower expectations to increase the impact of any success at the summit.

“The worst possible result at Copenhagen is a bad deal where the world leaders have to come home and say it’s a good deal when its rubbish,” he added.

“That’s the real danger – that they will feel under pressure to sign up to anything. That could lock us into something bad for the next ten years.”

Stewart Stevenson, Scotland’s climate change minister, who will also be attending the summit, said: “Even quite moderate predictions do suggest that we will have vast movements of people around the world particularly on the borders of desert regions and that associated with that will be loss of life.”